1. Field of the Invention
The present invention relates to an image forming apparatus for correcting registration deviation.
2. Description of the Related Art
As an image recording method used in a color image forming apparatus such as a color printer or a color copying machine, electrophotography is known. The electrophotography refers to a method of utilizing a laser beam to form a latent image on a photoconductive drum and performing development with charged color material (hereinafter referred to as “toner”). The recording of an image is performed by transferring an image developed with toner to transfer paper and fixing it.
In recent years, in order to increase the image formation speed of an electrophotographic color image forming apparatus, more and more tandem color image forming apparatuses have been used that have as many developing machines and photoconductive drums as the number of colors of toner and that sequentially transfer different color images on an image transport belt or a recording medium. In this tandem color image forming apparatus, a plurality of factors for causing registration deviation is known, and various methods of coping with each of the factors are proposed.
One of the factors is the nonuniformity or the fixing-position deviation of lens of a deflection scanning device, and attaching-position deviation of the deflection scanner against the main body of the color image forming apparatus. This position deviation causes a scanning line to be bent or curved, and the degree of the curve (hereinafter referred to as profile) varies from color to color, with the result that registration deviation occurs. The characteristic of the profile varies from image forming apparatus to image forming apparatus, that is, from recording engine to recording engine, and furthermore varies from color to color.
As a method of dealing with this registration deviation, there is a method of using an optical sensor to measure the inclination of a scanning line and the magnitude of the curve, correcting bitmap image data so as to perform cancellation and forming the corrected image (for example, see Japanese Patent Laid-Open No. 2004-170755). In this method, since processing is performed on image data to conduct electrical correction, a mechanical adjustment member and an adjustment step at the time of assembly are not necessary. Hence, it is possible to decrease the size of a color image forming apparatus and inexpensively deal with the registration deviation. This electrical registration deviation correction is divided into a one-pixel-basis correction and a less-than-one-pixel-basis correction.
The one-pixel-basis correction is performed by offsetting the pixel in the subscanning direction pixel by pixel according to the amount of correction of inclination and a curve. When this method is used, an image forming apparatus having a curve or inclination a few hundred to five hundred μm in size and a resolution of 600 dpi requires an image memory for a few tens of lines in order to perform the correction. In the following description, the position at which the pixel is offset is referred to as a changing point.
As shown in FIG. 19, the less-than-one-pixel-basis correction is performed by adjusting a tone value for image data with pixels in front and at the back in a subscanning direction. Specifically, when upward curving is present according to the profile, the image data before the correction is treated in the opposite direction to the direction indicated by the profile toward the side of the subscanning. By performing the less-than-one-pixel-basis correction in this way, it is possible to eliminate unnatural differences in height in the boundary of the changing point caused by the one-pixel-basis correction and thus smooth out the image.
However, in the method described above, the less-than-one-pixel-basis correction and the one-pixel-basis correction are constantly performed on an image after halftoning. Hence, unevenness in density disadvantageously occur depending on a halftoning method (for example, a processing method, such as error diffusion processing, by which to obtain a halftone image having a high spatial frequency).